<!--$Id: ram.so,v 10.11 2008/01/24 00:36:26 sarette Exp $-->
<!--Copyright (c) 1997,2008 Oracle.  All rights reserved.-->
<!--See the file LICENSE for redistribution information.-->
<html>
<head>
<title>Berkeley DB Reference Guide: Memory-only or Flash configurations</title>
<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
<meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++">
</head>
<body bgcolor=white>
<a name="2"><!--meow--></a>
<table width="100%"><tr valign=top>
<td><b><dl><dt>Berkeley DB Reference Guide:<dd>Programmer Notes</dl></b></td>
<td align=right><a href="../program/namespace.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../program/cache.html"><img src="../../images/next.gif" alt="Next"></a>
</td></tr></table>
<p align=center><b>Memory-only or Flash configurations</b></p>
<p>Berkeley DB supports a variety of memory-based configurations for systems
where filesystem space is either limited in availability or entirely
replaced by some combination of memory and Flash.  In addition, Berkeley DB
can be configured to minimize writes to the filesystem when the
filesystem is backed by Flash memory.</p>
<p>There are three parts of the Berkeley DB database environment normally written
to the filesystem: the database environment region files, the database
files and the database environment log files.  Each of these items can
be configured to live in memory rather than in the filesystem:</p>
<br>
<b>The database environment region files:</b><ul compact><li>Each of the Berkeley DB subsystems in a database environment is described by
one or more regions, or chunks of memory. The regions contain all of the
per-process and per-thread shared information (including mutexes), that
comprise a Berkeley DB environment.  By default, these regions are backed by
the filesystem.  In situations where filesystem backed regions aren't
optimal, applications can create memory-only database environments in
two different types of memory: either in the application's heap memory
or in system shared memory.
<p>To create the database environment in heap memory, specify the
<a href="../../api_c/env_open.html#DB_PRIVATE">DB_PRIVATE</a> flag to the <a href="../../api_c/env_open.html">DB_ENV-&gt;open</a> method.  Note that database
environments created in heap memory are only accessible to the threads
of a single process, however.</p>
<p>To create the database environment in system shared memory, specify the
<a href="../../api_c/env_open.html#DB_SYSTEM_MEM">DB_SYSTEM_MEM</a> flag to the <a href="../../api_c/env_open.html">DB_ENV-&gt;open</a> method.  Database
environments created in system memory are accessible to multiple
processes, but note that database environments created in system shared
memory do create a small (roughly 8 byte) file in the filesystem, read
by the processes to identify which system shared memory segments to
use.</p>
<p>For more information, see <a href="../../ref/env/region.html">Shared
memory regions</a>.</p></ul>
<b>The database files:</b><ul compact><li>By default, databases are periodically flushed from the Berkeley DB memory
cache to backing physical files in the filesystem.  To keep databases
from being written to backing physical files, pass the
<a href="../../api_c/memp_set_flags.html#DB_MPOOL_NOFILE">DB_MPOOL_NOFILE</a> flag to the <a href="../../api_c/memp_set_flags.html">DB_MPOOLFILE-&gt;set_flags</a> method.  This flag
implies the application's databases must fit entirely in the Berkeley DB
cache, of course.  To avoid a database file growing to consume the
entire cache, applications can limit the size of individual databases
in the cache by calling the <a href="../../api_c/memp_set_maxsize.html">DB_MPOOLFILE-&gt;set_maxsize</a> method.</ul>
<b>The database environment log files:</b><ul compact><li>If a database environment is not intended to be transactionally
recoverable after application or system failure (that is, if it will not
exhibit the transactional attribute of "durability"), applications
should not configure the database environment for logging or
transactions, in which case no log files will be created.  If the
database environment is intended to be durable, log files must either
be written to stable storage and recovered after application or system
failure, or they must be replicated to other systems.
<p>In applications running on systems without any form of stable storage,
durability must be accomplished through replication.  In this case,
database environments should be configured to maintain database logs in
memory, rather than in the filesystem, by specifying the
<a href="../../api_c/env_log_set_config.html#DB_LOG_IN_MEMORY">DB_LOG_IN_MEMORY</a> flag to the <a href="../../api_c/env_set_flags.html">DB_ENV-&gt;set_flags</a> method.</p></ul>
<br>
<p>In systems where the filesystem is backed by Flash memory, the number
of times the Flash memory is written may be a concern.  Each of the
three parts of the Berkeley DB database environment normally written to the
filesystem can be configured to minimize the number of times the
filesystem is written:</p>
<br>
<b>The database environment region files:</b><ul compact><li>On a Flash-based filesystem, the database environment should be placed
in heap or system memory, as described previously.</ul>
<b>The database files:</b><ul compact><li>The Berkeley DB library maintains a cache of database pages.  The database
pages are only written to backing physical files when the application
checkpoints the database environment with the <a href="../../api_c/txn_checkpoint.html">DB_ENV-&gt;txn_checkpoint</a> method,
when database handles are closed with the <a href="../../api_c/db_close.html">DB-&gt;close</a> method, or when
the application explicitly flushes the cache with the <a href="../../api_c/db_sync.html">DB-&gt;sync</a>
or <a href="../../api_c/memp_sync.html">DB_ENV-&gt;memp_sync</a> methods.
<p>To avoid unnecessary writes of Flash memory due to checkpoints,
applications should decrease the frequency of their checkpoints.  This
is especially important in applications which repeatedly modify a
specific database page, as repeatedly writing a database page to the
backing physical file will repeatedly update the same blocks of the
filesystem.</p>
<p>To avoid unnecessary writes of the filesystem due to closing a database
handle, applications should specify the <a href="../../api_c/db_close.html#DB_NOSYNC">DB_NOSYNC</a> flag to the
<a href="../../api_c/db_close.html">DB-&gt;close</a> method.</p>
<p>To avoid unnecessary writes of the filesystem due to flushing the cache,
applications should not explicitly flush the cache under normal
conditions -- flushing the cache is rarely if ever needed in a
normally-running application.</p></ul>
<b>The database environment log files:</b><ul compact><li>The Berkeley DB log files do not repeatedly overwrite the same blocks of the
filesystem as the Berkeley DB log files are not implemented as a circular
buffer and log files are not re-used.  For this reason, the Berkeley DB log
files should not cause any difficulties for Flash memory configurations.
<p>However, as Berkeley DB does not write log records in filesystem block sized
pieces, it is probable that sequential transaction commits (each of
which flush the log file to the backing filesystem), will write a block
of Flash memory twice, as the last log record from the first commit will
write the same block of Flash memory as the first log record from the
second commit.  Applications not requiring absolute durability should
specify the <a href="../../api_c/env_set_flags.html#DB_TXN_WRITE_NOSYNC">DB_TXN_WRITE_NOSYNC</a> or <a href="../../api_c/env_set_flags.html#DB_TXN_NOSYNC">DB_TXN_NOSYNC</a> flags
to the <a href="../../api_c/env_set_flags.html">DB_ENV-&gt;set_flags</a> method to avoid this overwrite of a block of
Flash memory.</p></ul>
<br>
<table width="100%"><tr><td><br></td><td align=right><a href="../program/namespace.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../program/cache.html"><img src="../../images/next.gif" alt="Next"></a>
</td></tr></table>
<p><font size=1>Copyright (c) 1996,2008 Oracle.  All rights reserved.</font>
</body>
</html>
